A typo in #165711 caused sanitizer failures (the small buffer was used
for the larger test). Renamed the variables to avoid the mistake in
future.

This patch fixes a bug in strftime's return value when the formatted
output exactly fills the buffer, not including the null terminator. The
previous check failed to account for the null terminator in this case,
incorrectly returning the written count instead of 0.

Implements the posix-specified strftime conversions for the default
locale, along with comprehensive unit tests. This reuses a lot of design
from printf, as well as the printf writer.

Roughly based on #111305, but with major rewrites.